Electrochemical doping of poly(3-octylthiophene) was studied by in situ optical absorption and in situ resonance Raman scattering. Absorption of polarons and, at higher doping levels, of bipolarons was observed. A strong decrease of luminescence and of main Raman lines was found particularly for the initial part of the doping process. The amplitude mode shifts about 30 cm-1 from the nearly undoped to the heavily doped state. This behaviour is attributed to the quenching of the Raman line of the undoped state and at higher doping levels to the simultaneous generation of the Raman line for the bipolaron configuration at lower wavenumbers.